1. Field of the Invention
This invention relates to a silicone composition of the addition curing type and more particularly, to such a silicone composition which is shelf stable at room temperature, quickly curable upon heating, and applicable in a variety of fields as potting agents, adhesives, and coating agents and in extrusion molding and liquid injection molding systems.
2. Prior Art
On heating, silicone compositions of the addition curing type are curable through hydrosilylation reaction in the presence of platinum compound catalysts. They are used in a multiplicity of applications because of the advantages that the curing reaction completes within a very short time and entails no by-products. Undesirably, these silicone compositions are unstable during shelf storage at room temperature. One countermeasure is to divide the composition into two parts for storage and combine them together on use, but it is unacceptable for practical use.
To overcome this drawback, a number of compounds capable of controlling a hydrosilylation reaction have been proposed. For example, U.S. Pat. No. 3,188,300 discloses organic phosphorus compounds, U.S. Pat. No. 3,445,420 or JP-B 31476/1969 discloses acetylene alcohols, U.S. Pat. No. 3,882,083 or JP-B 41626/1980 discloses triallylisocyanurates, U.S. Pat. No. 4,061,609 or JP-B 20340/1982 discloses hydroperoxides, U.S. Pat. No. 3,699,073 or JP-B 10947/1973 and U.S. Pat. No. 3,923,705 or JP-B 56563/1988 disclose high vinyl siloxanes. Silicone compositions having such control agents added thereto offer a sufficient pot life and curability as long as the compositions are used by conventional techniques. In the current market, more severe requirements are imposed on silicone compositions, that is, more satisfactory shelf stability and quick curing characteristics are required.
Encapsulation techniques are known to satisfy such requirements. In JP-B 41707/1978, a protected catalyst powder is prepared by mixing a platinum catalyst with a silicone resin having a melting point of 40.degree. to 200.degree. C. and pulverizing or spray drying the mixture. U.S. Pat. No. 4,481,341 or JP-A 37053/1983, U.S. Pat. No. 4,784,879 or JP-A 47442/1989 and 45468/1989 disclose a platinum catalyst encapsulated with a thermoplastic resin. The former technique cannot fully improve the shelf stability of silicone compositions. The latter technique of microcapsulation with organic resin is successful in enhancing the shelf stability of silicone compositions, but not in providing quick curing characteristics as required in the current market.
Therefore, there is a need to have a technique capable of further improving the shelf stability and curing characteristics of addition curing type silicone compositions.